If you don't have the original email as raw eml data, you won't be able to recreate it precisely.
To create a correct reply you need to have an IMail instance representing original email. You can use MailBuilder class to re-create it so it at least resembles the original.
MailBuilder originalEmailBuilder = new MailBuilder();
originalEmailBuilder.Subject = "subject";
originalEmailBuilder.MessageID = "message-id";
originalEmailBuilder.From.Add(new MailBox("original-from@example.com"));
originalEmailBuilder.To.Add(new MailBox("original-receipient@example.com"));
originalEmailBuilder.Text = "original text";
IMail original = originalEmailBuilder.Create();
ReplyBuilder replyBuilder = original.Reply();
replyBuilder.Html = "this is the <strong>reply</strong>";
var from = new MailBox("original-receipient@example.com");
MailBuilder builder = replyBuilder.Reply(from);
IMail reply = builder.Create();